Dual role of diffusion in tissue gas exchange: blood-tissue equilibration and diffusion shunt.

The role of diffusion in inert gas washout from tissue is investigated using simple mathematical models incorporating diffusive blood-tissue equilibration and diffusion shunt due to diffusive gas transfer between precapillary and postcapillary vessels with counter-current blood flow. With increasing diffusivity blood-tissue equilibration is improved, but simultaneously the diffusion shunt by veno-arterial back diffusion is increased. Similarly, with decreasing blood flow, the extent of diffusion limitation in blood-tissue transfer is diminished, but at the same time veno-arterial diffusive shunting is enhanced. Diffusion shunt slows inert gas washout (local tissue clearance) and thus, if not taken into account, leads to an underestimation of capillary blood flow calculated from the washout rate constant. Diffusion shunting of O2 diminishes the efficacy of blood-tissue transfer, but its extent is predicted to be smaller compared to that for inert gases, because the chemical combination of O2 in blood decreases diffusive shunting.